Electric vehicles and hybrid vehicles which use electric power from storage batteries as power are in widespread use. In such electric vehicles and hybrid vehicles, a DC-DC converter is used that is a power conversion device which boosts or steps down a direct-current (DC) voltage for charging and voltage conversion of the batteries. In particular, due to the demand for high efficiency and low noise, a power conversion device which employs current resonance has been recently used in electric vehicles and hybrid vehicles.
A DC-DC converter which employs current resonance is capable of performing zero voltage (current) switching, which operates the switching element in a state where at least one of voltage and current is zero, thereby reducing the power loss during the operation.
However, in the DC-DC converter which employs current resonance, the resonant frequency of at least one of a transformer and a resonant capacitor which are the components for resonance may vary due to aging degradation or a change in characteristics caused by temperature change in the transformer and/or the resonant capacitor. In such a case, zero voltage (current) switching cannot be accurately performed, which results in, for example, a decrease in power conversion efficiency due to an increase in switching loss, generation of switching noise, or circuit failure.
For example, Patent Literature (PTL) 1 discloses a technique for coping with variations in resonant frequency caused by the change in characteristics of the transformer and/or the resonant capacitor. PTL 1 discloses a power conversion device which resets the switching frequency only when a state, in which the input and output power are limited by an input and output power limiting means which limits the input and output power, continues for a predetermined time period.